Variable ratio transformer



Dec. 22, 1964 M. L. ROBERTS 3,162,799

VARIABLE RATIO TRANSFORMER Filed Jan. 30, 1961 L-O OUT 4 Fig. I

V I R 2 C F lg. 3 I R s T TT P I! INVENTOR. Q IF 4 Marion L. RoberfsBuck/70m, Chearham 3 Blore ATTORNEYS United States Patent 3,162,1 9?VAlltliAlilLE litraTilU TRANSFGRMER Marion L. Roberts, Portland, Greg,assignor to Gshorne Electronic Corporation, Portland, Greg, acorporation of Oregon Filed .lan. 3d, 1961, Ser. No. 85,744 8 Claims.(Cl. 323-45) This invention relates to electrical transformers ingenoral, and to variable ratio transformers in particular in which theoutput voltage is at least in part obtained from secondary windings andin which the input to output voltage ratio can be adjusted to any of alarge number of values in order to provide a selected ratio whose valuewill be precisely known. These precision transformers are useful inperforming many types of electrical test measurements and the like.

It is well known to provide transformers of the general type abovereferrend to in which the ratio of input voltage to output voltage canbe selected from a number of different values. When these types oftransformers are used for very precise measuring or controllingfunctions under no load conditions, their transformation ratios must beknown with an accuracy that is much greater than that of the ordinarytransformer. While the precision ratio transformers of the prior arthave been carefully constructed so that their primary windings andsecondary windings are evenly distributed and have the exact number ofturns required for a turns ratio which should result in the desiredtransformation ratio of input voltage to output voltage, this has notbeen the case because the resultant output voltage is not in phase withthe applied input voltage and is not equal in magnitude to the appliedvoltage multiplied by the turns ratio 'of thetransformer. The majorcause of this inaccuracy is due to the fact that even under no loadcondition the exciting current flowing in primary windings causes aninternal voltage drop due to the winding resistance and leakagereactance of the primary windings. The result is that the. voltageappearing across the secondary windings consists merely of the voltageinduced therein due to the mutual flux in the magnetic core of thetransformer so that it is les than the applied primary voltagemultiplied by the turns ratio and is also somewhat out of phase with theapplied primary voltage. This presents a problem when it is desired toemploy the secondary voltage, in whole or in part, to provide aplurality of output voltages which are accurately related to the appliedprimary voltage as fractions or multiples thereof.

In order to overcome this problem, the present inven tion makes use of acompensation impedance connected in series with the primary windings ofthe precision ratio transformer to add a compensating voltage, developedacross this compensation impedance by primary current flowingtherethrough, to the voltage induced in the secondary windings so thatthe output voltage of the transformer is more nearly in phase with itsinput voltage and the output voltage is more closely equal to the inputvoltage multiplied by the turns ratio of the transformer.

In one form, this compensation impedance is divided into a plurality ofimpedance steps corresponding in numher to a plurality of voltage stepsin the secondary windings of a transformer forming part of a precisionratio transformer apparatus. Selected portions of this compensationimpedance can be connected in series with selected portions of thesecondary windings of the. transformer so that a compensation voltage inphase with the voltage drop due to the impedance of the primary windingsis added to the induced voltage in such portions of the secondarywindings. For example, the impedance steps of the compensation impedancecan be selectively connected in series with the voltage steps of thesecondamass "ice o (in! ary windings by means of a switch ganged withthe voltage selection switch for the secondary windings. This enablesthe proper amount of compensating voltage having a correct phase angleto be added to each step of the induced secondary voltage. The vectorsum of such voltage is an output voltage which is more nearly in phasewith the applied input voltage of the transformer and more nearly equalto the applied voltage multiplied by the turns ratio.

In another form, a compensation impedance is connected in series withthe primary windings of a transformer forming a part of the variableratio transformer apparatus and the compensating voltage developedacr'oss such compensation impedance is added to the voltage developed inthe secondary windings of such transformer by means of anothertransformer having a proper turns ratio and having its primary windingsconnected across the compensation impedance and secondary windingsconnected in series with the secondary windings of the transformer towhich the compensation is being applied.

Therefore, one object of the present invention is to provide acompensation circuit which provides compensation to the secondary windins for the impedance voltage drop in the primary windings of a precisiontransformer.

Another object of the invention is to provide a variable ratiotransformer having a compensation impedance to correct the impropervoltage transformation, in the form of inaccurate magnitude ratios andphase shift of output voltage to input voltage, in the transformer dueto the internal voltage drop in the primary windings thereof.

A further object of this invention is to provide a precision transformerof the variable ratio type having a compensation impedance to correctfor the voltage drop in the primary windings of the transformer whensuch transformer is under a no load condition.

Still another object of the present invention is to provide a testcircuit for measuring the characteristics of transformers, or otherelectrical devices, by variable ratio transformers having compensationcircuits to correct for the voltage drop in the primary windings thereofso that they may be used as accurately calibrated standards in such testcircuit.

()ther objects and advantages of the present invention will be apparentwhen referring to the following detailed description of preferredembodiments of the invention, given by way of example and described inconnection with the attached drawings, of which:

FIG. 1 is a schematic diagram of the electrical circuit of a variableratio transformer in accordance with the present invention;

PEG. 2 is a vector diagram illustrating the relative magnitudes andphase relationships of various voltages and currents and the mutual fluxrelated to the primary windings of the variable ratio transformer ofFIG. 1;

FIG. 3 is a vector diagram of part of the output voltage from oneportion of the variable ratio transformer in PEG. 1; and

PEG. 4 is the schematic diagram of a test circuit in accordance with thepresent invention using the variable ratio transformer of HG. l.

The schematic diagram illustrated in FIG. 1 shows the electrical circuitused in the variable ratio transformer apparatus of the presentinvention and the voltage compensation circuit used therein. Thisapparatus includes five separate transformers, T T T T and T each havinga separate core of ferromagnetic material. Such transformers arearranged so that their output windings may be connected in series so thevoltage developed across each of these windings can be added thereby.This arrangement gives a final output voltage which is the sum of theseries added voltages. Step down transformer T, has primary windings Awhich are connected across a source of AC. voltage, shown as an A.C.line including conductors indicated by terms HIGH and LOW. Primarywindings A of transformer T are shown as being divided into ten equalvoltage steps which can be selectively connected by switch S in seriesbetween the line conductor marked LOW and the lower end of secondarywindings B positioned on a common magnetic core with primary windings AIt should be noted here that while each of the windings on step downtransformers T T and T have been shown to be divided into ten equalsteps each, that it is possible to divide these windings into any numberof equal or unequal voltage steps. It should be understood that the onlyreason for using a decade arrangement of ten equal steps is to enablethe utilization of the decimal system in computing the voltagetransformation ratio. Secondary windings B of step down transformer Talso have a decade arrangement so that they are divided into ten equalvoltage steps which are in turn selectively connectabie by switch S inseries between the switch S and lower end of secondary windings CSecondary windings C are similarly divided into a decade arrangement often equal steps which are selectively connectable by switch S in seriesbetween the switch S and the lower end of primary windings A oftransformer T Primary windings A of step down transformer T areconnected across secondary windings D of transformer T so that thevoltage across secondary windings D is used as the input voltage oftransformer T Primary windings A and secondary windings B C and D alongwith their associated switches, S S and S can be identical to thesimilar parts of transformer T Any number of step down transformerssimilar to T may be connected in series therewith in order to obtain thedegree of precision desired. The final step down transformer T isconnected with its primary winding A across secondary windings D oftransformer T so that the volt-age across secondary windings D serves asthe input voltage of transformer T Primary windings A also have a decadearrangement whose equal voltage steps are connectable by switch S7 inseries between switch S and the lower end of secondary windings B ontransformer T Secondary windings B are likewise provided with a decadearrangement of equal voltage steps connectable by means of switch S inseries between the switch S and the lower end of secondary windings B oftransformer T It will be apparent that the transformers T and T will beby-passed if the switch S is in its lowermost position 1X.

In this arrangement each step of primary windings A has one-tenth of theinput voltage V across its terminals while each step of secondarywindings B has induced therein somewhat less than one-tenth of thevdltage across each step of primary windings A or somewhat less thanone-hundredth of the input voltage V since there is a ten-to-one turnsratio between windings A and B Secondary windings C have one-tenth ofthe voltage developed across each step of secondary B induced in each ofits individual steps, or somewhat less than one-thousandth of the inputvoltage V since there is a similar ten-to-one turns ratio betweenwindings B and C The voltage developed across secondary windings D isequal to the voltage induced in one step of secondary windings C due toa ten-to-one turns ratio between windings C and D Each step of primarywindings A has induced therein one-tenth of the voltage across secondarywindings D or approximately one ten-thousandth of input voltage V; ofstep down transformer T In a similar manner secondary windings B haveinduced in each step thereof a voltage of approximately 10* V secondarywindings C have induced in each of its steps approximately 10* V eachstep of primary windings A has about 10* V induced therein, and finally,secondary windings B have induced in each of its steps about 10- V Thisis continued until the desired degree of precision is obtained in thefinal output voltage measured between switch S on transformer B and theLOW side of the line. The positions of the various switches associatedwith transformers T T and T in PEG. 1 result in an output voltage ofapproximately 036453564 times the input voltage V between switch S andthe LOW side of the line.

As shown in FIG. 2 the A.C. input voltage V applied to the primarywindings A of transformer T causes an A.C. current 1 to flow in theprimary windings which lags applied voltage V by phase angle 0. Under noload conditions the primary winding current I equals the excitingcurrent for the magnetic core of the transformer. This exciting currentI may be resolved into two components, a magnetizing current 1 and acore loss current 1 in quadrature therewith. Magnetizing current 1produces a mutual flux which links with the primary windings A andsecondary windings B of the transformer. This mutual flux at induces avoltage E in primary winding A which lags the mutual flux by degrees.The core loss current I supplies the energy for the hysteresis andeddy-current losses in the magnetic core of the transformer T Whenexciting current I flows through primary A the winding resistance ofthis primary causes a voltage drop I R which is in phase with theexciting current I Since the leakage flux and its associated leakagereactance can be neglected in a well-designed toroidal transformer, theapplied voltage V can be considered to be the vector sum of the inducedvoltage E and the winding resistance voltage drop I R The voltagedeveloped across the secondary windings B when no load is applied tosuch secondary windings consists only of the voltage E which is inducedin secondary windings B by mutual flux as shown in FIG. 3. This inducedvoltage E is in phase with the induced voltage E; of primary windings Asince primary windings A and secondary windings B are on the samemagnetic core and are linked by the same mutual flux. Since no currentflows in the secondary windings B due to its no load condition, novoltage drop due to the internal resistance or leakage reactance of thewindings B is produced. Therefore, the voltage developed acrosssecondary windings B is merely the induced voltage E which is out ofphase with the voltage V applied to primary windings A by the phaseangle a and is not equal to such voltage multiplied by the turns ratioof windings A and B the magnitudes of such phase angle on and thevoltage R being exaggerated in FIG. 2. That is to say, increments of theinduced secondary voltage E do not provide output voltage steps whichare in phase with or which have a magnitude equal to the applied voltageV multiplied by the turns ratio so that an inaccuracy occurs.

In order to compensate for the inaccuracy discussed above selectedportions of a compensation impedance, which may be suitable tappedresistor R are connectable in series with primary A by means of a switchS ganged with the switch S for secondary windings B Compensationresistor R is divided into a plurality of resistance steps correspondingin number to the voltage steps of secondaries B put voltage of eachselected step of secondary windings B and the corresponding step ofcompensating resistor R consists of a fraction E of the induced voltageE of secondary windings B and a voltage l R which very closelyapproaches the fraction of 1 R required to produce a resulting outputvoltage step in phase with the input voltage V and of the desiredmagnitude. value of each resistance step of compensating resistor R isselected so that it is proportional by the turns ratio to the windingresistance of each voltage step in primary A primary current 1 flowingthrough this compensation resistance produces a voltage drop in phasewith and proportional to the resistance voltage drop in the windings Asa result the combined out- Since the.

of primary A When the total compensating resistance voltage l R is addedto the total induced voltage E of the secondary winding 13;, theresultant voltage V; is obtained as shown in FIG. 3 which is in phasewith the primary voltage V applied to the primary A and more nearlyequal. to the applied voltage nultiplied by the turns ratio. The samerelation holds for each step of the windings B and compensatingresistance R as indicated by the voltages E and f R of FIG. 3. It shouldbe noted that the resistance of compensation resistor R is smallcompared to the impedance of primary A so that its effect upon themagnitude and phase angle of applied voltage V can be neglected.

Since step down transformers T T and T are connected with their outputwindings in series, the voltages developed across each of their windingsare added. it is apparent that voltage compensation similar to thatdiscussed above could be applied to each of the remaining windings ofthese transformers to obtain an even more accurate transformation. Thisis not done in practice because the voltages developed across theremaining windings are very small with respect to the voltage developedacross primary A so that such compensation is, in general, notnecessary.

it is often desirable, however, to add a step up transformer T to thevariable ratio transformer apparatus in order to add a multiple of theline voltage tothe output voltage of the transformer apparatus. Whenthis is done a compensating impedance in the form of a resistor R isconnected in series with the primary windings A of thestep uptransformer T since the secondary windings B and C are operated under noload condition and the volt-ages induced therein are less than theapplied voltage times the turns ratio because of the resistance voltagedrop in primary windings A Unlike compensating resistor R thecompensating resistor R is not divided into a plurality of resistancesteps, but the voltage drop across this compensating resistance isstepped up and added to the voltage induced in secondary windings B andC by means of a compensating transformer T A step up compensatingtransformer is required since it is necessary to add a voltage to theoutput voltage of each of the secondary windings B and (1, which isequal to the IR drop in primary windings A and compensating resistor RThis transformer T has its primary windings A connected across thecompensating resistor R and has its secondary windings B and C connectedin series with secondary windings C and B respectively of step uptransformer T When switch S has its movable contact in the lowerposition IX in FlG. l transformers T and T a e bypassed and no multipleof the line voltage is added to the output of the variable ratiotransformer apparatus. Also the switch S is ganged with the switch S todisconnect transformer T,; from the line when such switches are in theposition lX. However, wh n switches S and S are in the position 2X, asshown in FIG. 1, the step up transformer T4- is connected to the lineaudits secondary windings C are connected-in series withthe secondarywindings E through secondary windings B of the transformer T so that acompensated voltage, which is a multiple of the linevoltage is added tothe output voltage from windings B If there is a one-tonne turns ratiobetween primary winding A and each of the secondary windings B and Cthis added voltage will be equal to the line voltage. When switches Sand S are in the position 3X, both secondary windings B and C areconnected in series with the output from secondary windings B; throughsecondary windings B and C of transformer T resulting in a compensated,voltage equal to two times the line voltage being added to the outputfrom secondary windings B The turns ratio of compensating transformer Tbetween primary windings A and each of the secondary windings B and C isselected so that the compensating voltage developed across compensatingresistor R is properly transformed in order that the magnitude and phaseangle of the compensating voltage so transformed corresponds to that ofthe resistance voltage drop in primary windings A and R This variableratio transformer with its compensation means may be used to test anyelectrical device in any test circuit where the transformer is usedunder no load condition. One such test circuit is shown in FIG. 4 bywhich other transformers may be tested for their accuracy of voltagetransformation. The primary windings of the transformer T T being testedare connected across an AC. voltage source E. A variable ratiopercentage transformer T is connected with its primary windings inparallel with the primary windings of test transformer T The compensatedvariable ratio transformer T of FIG. 1, shown as an auto transformer inFIG. 4 for simplicity, is used as a comparison standard by having oneend of its windings connected directly to the low side LO of the line,and a selected tap of the primary windings connected in series with thesecondary windings of the percentage transformer T through switch S tothe high side HI of the line. The secondary windings of test transformerT have one end connected directly to the low side of the line, and theother end by switch S to a selected voltage output tap on the windingsof the standard transformer T through an electrical meter M. Thesecondary windings of the test transformer T and the windings ofstandard transformer T are connected to have their voltages oppose orbuck each other so that the standard transformer T is operated under noload conditions.

When testing for the turns ratio or the accuracy of the voltagetransformation ratio of the transformer T being tested, the meter Mconnected in series with the secondary windings of test transformer andthe standard transformer is a voltage indicator. The desired voltagetransformation ratio may then be set on the compensated variable ratiotransformer T which is used as a standard because of its precision andaccuracy. The contact switch S on percentage transformer T is then moveduntil a minimum reading is obtained on the voltage indicator M. Sincethe secondary windings of percentage transformer T are divided into aplurality of percentage steps, the setting of switch S on such secondarywindings is then read to determine how accurate the transformer undertest is compared to the standard transformer T The reading on thepercentage transformer T given by the setting of switch S can be termsof the percentage deviation of the transformer under test T from thedesired turns ratio set on variable ratio transformer T It is alsopossible to determine the actual transformation ratio of the testtransformer T directly from the reading given by switch S on standardtransformer T In order to obtain this actual ratio directly switch S isset at zero on percentagetransformer T p and the switch S oil-standardtransformer T is moved until a minimum voltage reading appears onvoltage indicator M.

It is also possible to accurately determine the phase shift between theinput and output voltages of the transformer T under test by using thetest circuit shown in FIG. 4 when a phase angle meter is substituted forthe voltage indicator and connected in the circuit to show the phaseangle between the voltage across the test transformer T and that acrossthe terminals connected to the meter M in FIG. 4. The output voltage ofpercentage transformer T is set to zero voltage by switch S and thestandard transformer T is adjusted to produce a phase angle reading ofthe phase meter. This indicates that the voltage across the terminalsshown connected to the meter M in FIG. 4- is 90 out of phase with thevoltage developed across the output of standard transformer T whichvoltage is very nearly in phase with the input voltage of transformer Twith its compensation means. Since at this time the same input voltageis across the primary of both transformers T and T the output voltage oftransformer T is very nearly in phase with the input voltage oftransformer T Next the switch S on percentage transformer T is movedfrom its zero position until the reading of the phase meter is 45. Itcan then be shown that the output voltage of transformer T divided bythe output voltage of transformer T s is the tangent of the angle ofphase difference between the output voltages of transformers T and TSince the phase shift of standard transformer T is very small or zerowith its compensation, this angle is also the phase angle between theinput and output voltages of the transformer under test T It should benoted that the frequency of AC. voltage source B should be near that ofthe rated frequency of the transformers used in the test circuit of FIG.4 and that the percentage transformer T may have a compensation meanssimilar to that used in standard transformer T While the presentinvention has been described above in connection with certain specificembodiments thereof, it should be understood that various modificationsas to detail will occur to those skilled in the art and that suchmodifications which would be obvious to one having ordinary skill areencompassed by the concept of the present invention. Therefore, thescope of the present invention is not limited to the embodimentsillustrated, but is defined only in the following claims.

I claim:

l. A precision electrical transformer comprising:

a magnetic core,

primary and secondary windings on said core,

compensation means connected to said secondary winding to produce underno load conditions an output voltage on said secondary winding which issubstantially in phase with the input voltage applied to said primarywinding and equal to the product of said input voltage multiplied by theturns ratio between said primary and secondary windings,

said compensation means including means to produce a compensatingvoltage in phase with and equal to the product of the impedance voltagedrop in said primary windings multiplied by the turns ratio between saidprimary and secondary winding and to add said compensating voltage tothe voltage induced in said secondary winding under no load conditions.

2. A precision electrical transformer comprising:

a magnetic core,

primary and secondary windings on said core,

a compensation impedance connected in series with said primary windingto provide under no load conditions a compensation impedance voltagedrop across said compensation impedance in phase with and proportionalto the impedance voltage drop in said primary winding due to currentflowing in said pr-imray winding, and

means for connecting said compensation impedance to the secondarywinding to add to the voltage induced in said secondary winding under noload condition a compensating voltage derived from said compensationimpedance voltage drop which is in phase with the impedance voltage dropin said primary winding and equal to the product of said impedancevoltage drop in said primary winding multiplied by the turns ratiobetween said primary and secondary windings.

3. A precision electrical transformer comprising:

a core of magnetic material,

primary windings on said core,

secondary windings on said core divided to provide a plurality of firstvoltage steps,

a compensation impedance connected in series wit said primary windingsand divided to provide a plurality of second voltage steps,

a switch to connect a selected number of said first voltage steps inseries with the output of said transformer, and

Cit

a second switch to connect a corresponding number of said second voltagesteps in series with said secondary winding and the output of saidtransformer so that the effective open circuit output voltage of saidecondary windings is equal to the sum of the voltage across saidselected steps of said secondary windings and the voltage across saidselected steps of said compensation impedance.

4. A precision electrical transformer comprising:

a core of magnetic material,

primary windings on said core,

secondary windings on said core,

compensation resistance connected in series with said primary windings,

first switch means connected between said primary and said secondarywindings to divide said primary windings into a plurality of equalvoltage steps,

a second switch means connected between said secondary windings and itsoutput to divide said secondary windings into a plurality of equalvoltage steps,

third switch means connected between said primary windings and saidcompensation resistance to divide said compensation resistance into aplurality of resistance steps and to connect it in series with saidsecondary windings so that the effective open-circuit output voltage ofeach step of said secondary windings is equal to the sum of the voltageinduced in the last mentioned step of said secondary winding by thevarying magnetic flux in said core due to the AC. current flowing insaid primary windings plus the voltage drop developed across thecorresponding step of said compensation resistance by said primarywinding current.

5. A precision electrical transformer comprising:

a core of magnetic material,

primary windings on said core,

secondary windings on said core,

compensation impedance means connected in series with said primarywindings,

a first switch connected between said primary windings and saidsecondary windings to divide said primary windings into a plurality ofequal voltage steps,

a second switch connected between said secondary windings and the outputof said transformer to divide said secondary windings into a pluralityof equal voltage steps,

a third switch connected between said primary windings and saidcompensation impedance to divide said compensation impedance into aplurality of impedance steps and to connect it in series with saidsecondary windings so that the effective open-circuit output voltage ofeach step of said secondary windings is equal tothe sum of the voltageinduced in eachstep of said secondary windings by the varying magneticflux in said core due to the AC. current flowing in said primary and thevoltage drop developed across each step in said compensation impedanceby said primary current, said second and third switches beingmechanically ganged so that each voltage step of said secondary windingsis coupled with a separate impedance step of said compensation impedancein order to compensate for the imedance voltage drop in said primarywindings.

6. A precision electrical transformer apparatus com:

a plurality of transformers each having a core of magnetic material,primary windings on said core and a plurality of secondary windings onsaid core,

compensation impedance connected in series with the primary windings ofthe first of said transformers,

a first switch connected between the primary windings and the firstsecondary windings of said first transformer to divide said primarywindings into a plurality of equal voltage steps,

a second switch connected between said first secondary a compensationtransformer connected with its primary the secondary windings of saidstep up transformer in order to correct for the impedance voltage dropin the primary windings of said step up transformer.

into a plurality of equal voltage steps, 8. A precision electricaltransformer apparatus coma third switch connected between said primarywindprising:

ings and said compensation impedance to divide said a plurality ofvoltage step down transformers each compensation impedance into aplurality of steps having a core of magnetic material, primary windandto connect it in series with said first secondary ings on said core anda plurality of secondary windwindings so that the effective open-circuitoutput ings on said core, voltage of each step of said first secondarywindings 9 a first compensation impedance connected in series is equalto the sum of the voltage induced in each with the primary windings ofthe first of said step step of said first secondary windings plus thevoltage down transformers, drop developed across each step of saidcompensaa first switch connected between said primary windings tionimpedance by the electrical current flowing in and first secondarywindings ofsaid first transformer said primary, each of the remainingwindings of to divide said primary windings into a plurality of saidfirst transformer and the remaining transformers equal voltage steps,being connected in series by switches which divide a second switchconnected between said first secondary each of them into a plurality ofequal voltage steps windings and other secondary windings on said firstin order to add the output voltages of said windings transformer todivide said first secondary windings and obtain a final output voltagehaving a high de into a plurality of equal voltage steps, gree ofprecision. a third switch connected between said primary wind- 7. Aprecision electrical transformer apparatus comh1g8 and Said first pe tiimpe ance to divid prising: said first compensation impedance into aplurality of a plurality of voltage step down transformers each hav-Steps and t0 Connect it in 56165 With Said first ing a core of magneticmaterial, primary windings (mdfiry ndings So that the effectiveopen-circuit on said core and a plurality of secondary windings OutputVoltage of each p of Said first Secondary on said core, windings isequal to the sum of the voltage induced first compensation impedanceconnected in series in each p of Said first secondary windings by thewith the primary windings of the first of said step Varying magneticflux in Said Core P the Voltage down tran former drop developed acrosseach step of said first coma first switch connected between the primarywindings Phhsahoh impcdahcfi y U16 electrical Current and firstsecondary windings of said first transihg in Said P y windings, each ofthe Ifimainiflg former to divide said primary windings into apluwindings 0f Said first transformer and the remaining rality f equalVoltage steps, step down transformers being connected in series by asecond switch connected between said first secondary Switches whichdivide each of them into a plurality windings and other secondarywindings on said first of equal Voltage Steps in Order to add the Outputtransformer to divide said first secondary windings Voltages of saidremaining windings and Obtain an into a plurality of equal voltagesteps, Output Voltage having a high degrfie of Precision, third switchconnected between said primary wind- 21 Vohagfi p p transformer With itsp y windings ings and said first compensation impedance to divideCOhheCthd in Parallel With the p 'y windings and said first compensationimpedance into a plurality of first Compensation impedance of Said firstp down steps and to connect it in series with said first sectransformer,and having its Secondary windings C011- ondary windings so that theeffective open-circuit hected in Series with the output of the p downoutput voltage of each step of said first secondary transformers sotheir Output Voltages are addhd windings is equal to the sum of thevoltage induced thereby, i h t f id fi t Secondary i d by the a secondcompensation impedance connected in series varying magnetic flux in saidcore plus the voltage With the Primary windings of Said P P transdropdeveloped across each step of said first comformeh and pensationimpedance by the electrical current flowing a co'mphhsatioh trahsfhrmelCohhhcted With its P in said primary windings, each of the remainingmerywindinssin Parallelwithsaidsewnd compensawindings of said firsttransformer and the remaintioh impedance and its Secondary windings inSeries ing step down transformers being connected in series With theSecondary windings of said p P tra11$- b i h hi h divide each f theminto a former in order to correct for the impedance voltage rality ofequal voltage steps in order to add the drop in the Primary windings ofSaid Step P trans output voltages of said remaining windings andobfohheh and wi an Output voltage having a high degree of mg a sw tchmeans connected to the secondary winding of i i said step up transformerto change the ratio of the a step up voltage transformer with itsprimary windings voltage Output to the Voltage input of Said Step Pconnected in parallel with the primary windings and tmhsfcmlerfirstcompensation impedance of said first ste down transformer and having itssecondary windings con- References Cited by the Exammer nected in serieswith the output of the last step down UNITED STATES PATENTS gansformerso their output voltages are added there- 2,667,617 1/54 Boyajian Y, i2,891,214 6/59 Rogers 323-44 a Second compensatlon im pedance connectedin series 2,396,156 7/59 Perrins 324 55 with the primary windings ofsaid step up trans- 2 911 91 11 59 Pritchgtt 324 55 f and 3,040,240 6/62Gotal et a1 323-4 09 X LLOYD MCCOLLUM, Primary Examiner.

MILTON O. HIRSHFIELD, Examiner.

8. A PRECISION ELECTRICAL TRANSFORMER APPARATUS COMPRISING: A PLURALITYOF VOLTAGE STEP DOWN TRANSFORMERS EACH HAVING A CORE OF MAGNETICMATERIAL, PRIMARY WINDINGS ON SAID CORE AND A PLURALITY OF SECONDARYWINDINGS ON SAID CORE, A FIRST COMPENSATION IMPEDANCE CONNECTED INSERIES WITH THE PRIMARY WINDINGS OF THE FIRST OF SAID STEP DOWNTRANSFORMERS, A FIRST SWITCH CONNECTED BETWEEN SAID PRIMARY WINDINGS ANDFIRST SECONDARY WINDINGS OF SAID FIRST TRANSFORMER TO DIVIDE SAIDPRIMARY WINDINGS INTO A PLURALITY OF EQUAL VOLTAGE STEPS, A SECONDSWITCH CONNECTED BETWEEN SAID FIRST SECONDARY WINDINGS AND OTHERSECONDARY WINDINGS ON SAID FIRST TRANSFORMER TO DIVIDE SAID FIRSTSECONDARY WINDINGS INTO A PLURALITY OF EQUAL VOLTAGE STEPS, A THIRDSWITCH CONNECTED BETWEEN SAID PRIMARY WINDINGS AND SAID FIRSTCOMPENSATION IMPEDANCE TO DIVIDE SAID FIRST COMPENSATION IMPEDANCE TODIVIDE STEPS AND TO CONNECT IT IN SERIES WITH SAID FIRST SECONDARYWINDINGS SO THAT THE EFFECTIVE OPEN-CIRCUIT OUTPUT VOLTAGE OF EACH STEPOF SAID FIRST SECONDARY WINDINGS IS EQUAL TO THE SUM OF THE VOLTAGEINDUCED IN EACH STEP OF SAID FIRST SECONDARY WINDINGS BY THE VARYINGMAGNETIC FLUX IN SAID CORE PLUS THE VOLTAGE DROP DEVELOPED ACROSS EACHSTEP OF SAID FIRST COMPENSATION IMPEDANCE BY THE ELECTRICAL CURRENTFLOWING IN SAID PRIMARY WINDINGS, EACH OF THE REMAINING WINDINGS OF SAIDFIRST TRANSFORMER AND THE REMAINING STEP DOWN TRANSFORMERS BEINGCONNECTED IN SERIES BY SWITCHES WHICH DIVIDE EACH OF THEM INTO APLURALITY OF EQUAL VOLTAGE STEPS IN ORDER TO ADD THE OUTPUT VOLTAGES OFSAID REMAINING WINDINGS AND OBTAIN AN OUTPUT VOLTAGE HAVING A HIGHDEGREE OF PRECISION, A VOLTAGE STEP UP TRANSFORMER WITH ITS PRIMARYWINDINGS CONNECTED IN PARALLEL WITH THE PRIMARY WINDINGS AND FIRSTCOMPENSATION IMPEDANCE OF SAID FIRST STEP DOWN TRANSFORMER, AND HAVINGITS SECONDARY WINDINGS CONNECTED IN SERIES WITH THE OUTPUT OF THE STEPDOWN TRANSFORMERS SO THEIR OUTPUT VOLTAGES ARE ADDED THEREBY, A SECONDCOMPENSATION IMPEDANCE CONNECTED IN SERIES WITH THE PRIMARY WINDINGS OFSAID STEP UP TRANSFORMER, AND A COMPENSATION TRANSFORMER CONNECTED WITHITS PRIMARY WINDINGS IN PARALLEL WITH SAID SECOND COMPENSATION IMPEDANCEAND ITS SECONDARY WINDINGS IN SERIES WITH THE SECONDARY WINDINGS OF SAIDSTEP UP TRANSFORMER IN ORDER TO CORRECT FOR THE IMPEDANCE VOLTAGE DROPIN THE PRIMARY WINDINGS OF SAID STEP UP TRANSFORMER, AND A SWITCH MEANSCONNECTED TO THE SECONDARY WINDING OF SAID STEP UP TRANSFORMER TO CHANGETHE RATIO OF THE VOLTAGE OUTPUT TO THE VOLTAGE INPUT OF SAID STEP UPTRANSFORMER.